Synthesis, conformational analysis, and spectroscopic characterization of peptides based on Daf, the first rigid transition-metal receptor, cyclic C-alpha, alpha-disubstituted glycine

Three series of terminally protected model oligopeptides to the nonamer level, based on 9-amino-4,5-diazafluorene-9-carboxylic acid (Daf) and either Gly, L-Ala, or Aib were synthesized. The molecular structures of two derivatives and one tripeptide were determined in the crystal state by X-ray diffraction. The solution preferred conformations of these peptides were assessed by FT-IR and NMR techniques. A spectroscopic (uv absorption, fluorescence, CD) of this novel heteroaromatic C-alpha-alpha-disubstituted glycine is also reported. Preliminary conformational data and membrane activity are reported for an analog of the lipopeptaibol antibiotic trichogin GA IV in which a Daf residue was incorporated at position 1 (replacing the original Aib).